1. Field of the Invention
The present invention concerns the storing of information in binary form with a view to using this information by various data processing or video systems making use of such memories.
2. Description of Related Art
Generally speaking, these memories, known as magneto optic memories, are described, for example, in the following publications: "Amorphous transition metal-rare earth alloy films for magneto optic recording" (Fred E. Luborsky, General Electric Corporate Research and Development, Mat. res. soc. symp. Proc. Vol. 80, 1987, p.375) and "Les Nouvelles Techniques de Stockage de Donnees" (New Techniques for Data Storage) (For "La Science," Dec. 1987, p.64).
First of all, there follows a description of the known general principle of such magneto optic memories with reference to the diagrams of FIGS. 1 and 2, the latter figure representing the magneto optic disk of FIG. 1 on larger scale.
The device of FIG. 1. mainly comprises a disk made of a magneto optic material 2 able to rotate about its vertical axis XY and a read/write system comprising a laser diode 4, a polarizer 6, an analyzer 8 and a detector 10. A semi-transparent mirror 12, slanted at 45.degree., is inserted in the path of the light which moves from the laser diode 4 as far as the surface of the magneto optic disk 2. Focussing lenses 14 are also provided close to the magneto optic disk 2. Then the writing and reading of the disk 2 is carried out with the aid of the light of the laser diode 4 which is able to write and read the data on the erasable and rerecordable magnetic support 2. To this effect, and for writing, a magnetic field coil 16 is also provided under the disk 2 so as to subject the latter to an intense magnetic field with a vertical axis, that is, in this case, perpendicular to the plan of the disk 2. The magnetic material constituting the disk 2 has an extremely intense coercive field at ambient temperature, but which reduces rapidly at high temperatures. Accordingly, when under the action of the laser beam emitted by the diode 4, a write domain of the disk 2 is heated intensely, the coercive field reduces drastically in this domain so that the magnetic field, emitted by the coil 16 and to which the disk is subjected, makes it possible to magnetically polarize the heated part and to give the domain in question a direction perpendicular to the surface of the disk 2, this direction being, however, a specific and known direction. Thus, it can be readily understood that if a magneto optic disk is used which has bee previously magnetized in a uniform direction perpendicular to the surface (FIG. 2) the implementation of the diagrammatic device of FIG. 1 enables the magnetized domains to be recorded side by side in a direction perpendicular to the disk 2, but whose magnetization is sometimes directed towards the lower face and sometimes directed towards the upper face (18 and 20, FIG. 2) In this way, the writing of the two binary digits 0 and 1 are embodied on the surface of the disk 2. FIG. 2 shows the support 22 made of glass and a reflecting film 24 which complete the structure of the optical memory disk.
In order to read this disk, resort is made to either the known Kerr effect or the Faraday effect. In other words, each of the domains is observed with the aid of the lenses 14 and a semi-transparent mirror 12 system by seeking with the aid of the analyzer 8 and the detector 10 whether the polarization plane to which the incident light has been subjected by the polarizer 6 has turned or not and in what direction inside the reflected beam. By thus detecting the rotations of the polarization plane of the radiation reflected onto the film (Kerr effect) or reflected after crossing the film (Faraday effect), for each analyzed domain, a determination is made as to whether this corresponds to the recording of a binary digit 0 or a binary digit 1.
With respect to the information storage memories with the aid of magnetic materials, such as Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, FeNi, etc., the optical memories present a certain number of characteristics rendering them to be more high performing and thus more advantageous. These characteristics include a storage capacity at least ten times greater than the storage capacity of magnetic memories, allow for easy optical reading and a reading head which is spaced from the disk (such as about one millimeter), which significantly reduces the risk of deterioration of the surface of the memories as is the case with currently existing magnetic memories. Furthermore, optical memories are relatively insensitive to dust and may be embodied on disks or movable supports.
Most of the disks or magneto optic memories produced up until now are made up of amorphous materials of the "rare earth/transition metal" type, such as compound Gd-Tb-Fe.
The Applicants have found that it was possible to further increase the performance of magneto optic memories by using oxides (ferrimagnetic garnets, hexaferrites, spinel ferrites) as a sensitive material, provided they are disposed on the support in a crystalline form. In accordance with these conditions, the preceding oxides possess the following advantages:
they possess extremely good chemical resistance, whereas amorphous metallic ones are oxidizable;
their magneto optic properties are greater (Faraday rotation is more than 5.times.10.sup.6 degrees of rotation per meter of sensitive film traversed);
it is also possible to use them with shorter wavelengths and accordingly obtain a greater recording density. For example, it is relatively simple to work with such memories with the aid of light of a wavelength of 500 nanometers.
Finally, their solidity does not require the presence of a protection film.
However, the production of such crystallized oxide memories, whose crystallization temperature is extremely high (T.sub.c &gt;500.degree.C.), poses the problem of the compatibility of the support which, during the heating time required to produce crystallization, needs to resist temperatures as high as above.